In recent years, the intensification of rainfall caused by climate change has become a global problem. Many earthfill dams in Japan have weak embankments built more than a century ago, raising concerns about seepage and sliding failure during heavy rainfall. The authors have thus concentrated on the use of double steel sheet pile walls to reinforce structures against heavy rainfall. For practical application, the soil strength of the embankment must be assessed during the design phase. For this purpose, it is necessary to understand the infiltration behavior of rainfall and overflow water, which was investigated in this study using model experiments and numerical analysis. The results showed that reinforcing the embankment with steel sheet piles was effective in delaying the saturation of the downstream slope in terms of rainfall infiltration through the embankment surface. It was also confirmed that overflow infiltration from the bottom of the slope occurred gradually and had no negative effect from the installation of the steel sheet piles. The findings of this study are expected to help improve the heavy rainfall protection of earthfill dams, thus facilitating the development of infrastructure for agricultural activities that can withstand the effects of climate change.